The present invention relates to an image forming apparatus using a rotary developing unit having a plurality of developing sections therein and, more particularly, to an image forming apparatus capable of accurately positioning the developing sections, or movable bodies, as to the rotation angle.
A copier, facsimile apparatus, printer or similar image forming apparatus includes a photoconductive element, or image carrier, and a developing device facing the element. The developing device is often implemented as a rotary developing device, or revolver as generally referred to, having a plurality of developing sections therein. The developing sections each stores a toner of particular color. The revolver is rotated to bring one of the developing sections to a developing position where it faces the photoconductive element. At the developing position, the developing section develops a latent image formed on the photoconductive element by the toner thereof. This kind of developing device is disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 62-251772 and 63-78170 and Japanese Utility Model Laid-Open Publication No. 63-41164. The prerequisite with the revolver is that the developing sections be sequentially brought to and accurately positioned at the developing position. For this purpose, it has been proposed to form recesses, each matching the respective developing section, in the outer periphery of the rear end wall or similar wall of the revolver, and to mount a roller capable of falling in one of the recesses at a time on a predetermined position of the apparatus body. The revolver is rotated by an amount necessary for one of the recesses matching the developing section to be used to face the roller. As a result, the roller falls in the recess and thereby accurately positions the revolver.
However, with the revolver of the type described, it is likely that the necessary amount of rotation cannot be achieved due to irregularities in a motor assigned to the revolver and changes in load. As a result, the roller fails to mate with the expected recess, i.e., to accurately position the revolver. It follows that the distance between a developing roller disposed in the developing section brought to the developing position and the photoconductive element differs from a target distance, resulting in poor image quality. This is also true with other various movable bodies arranged in the image forming apparatus and needing accurate positioning.